Method of upscaling a discrete fracture network model

Abstract

A discrete fracture network model is upscaled to a simulation grid having effective permeabilities for each grid cell. Prior to computing the effective permeabilities, the grid cells are grouped in distinct grid cell clusters, such that flow via fractures is only possible between grid cells that mutually belong to the same grid cell cluster. This is achieved by grouping fractures into distinctive fracture clusters, whereby all fractures that are physically connected with each other by intersection, either directly or indirectly via a number of other physically connected fractures, exclusively belong to one fracture cluster. Each grid cell is assigned to exclusively one fracture cluster. After defining the grid cell clusters, effective permeabilities are calculated for each grid cell using only the fractures of the fracture cluster to which the grid cell is assigned while fractures from other fracture clusters are ignored. Inter-cluster flow impediment data is assigned to selected grid cells.

What is claimed is: 
     
       1. Method of upscaling a discrete fracture network model, comprising:
 providing a discrete fracture network model of a subsurface fractured hydrocarbon bearing reservoir formation, said discrete fracture network model comprising a plurality of fractures; 
 grouping all fractures of the plurality of fractures into fracture clusters, whereby all fractures that are physically connected with each other by intersection, either directly or indirectly via a number of other physically connected fractures, exclusively belong to one fracture cluster such that no fracture within one fracture cluster is physically connected with any fracture of any of the other fracture clusters; 
 assigning a unique fracture cluster number to each of the fracture clusters; 
 overlaying a three dimensional simulation grid over the discrete fracture network model, consisting of a three dimensional array of grid cells in three directions; 
 attributing a grid cell cluster number to each grid cell in the simulation grid, which grid cell cluster number uniquely maps to the unique fracture cluster number of exclusively one fracture cluster that intersects with the grid cell; 
 computing an effective permeability for each grid cell and for each of the three directions using the fractures that are inside the grid cell and that form part of the fracture cluster that has the fracture cluster number to which the grid cell cluster number of the grid cell is uniquely mapped, whereby ignoring other fractures that are inside the grid cell but that form part of a fracture cluster having a fracture cluster number that does not map to the grid cell cluster number of the grid cell; and 
 assigning inter-cluster flow impediment data to selected grid cells reflecting flow impediment bathers between neighboring grid cell clusters; 
 outputting a table containing grid property data for all grid cells, wherein the grid property data for each grid cell comprises the cluster number assigned to the grid cell, and one effective permeability number for each of the three directions; and 
 outputting inter-cluster flow impediment data. 
 
     
     
       2. The method of  claim 1 , wherein when more than one fracture cluster intersect with the grid cell the grid cell cluster number attributed to the grid cell maps to the unique fracture cluster number of the one of the fracture clusters that intersects with the grid cell and that intersects with the highest number of other grid cells compared to how many other grid cells intersect with the other fracture clusters that intersect the grid cell. 
     
     
       3. The method of  claim 1 , wherein the grid property data for each grid cell further comprises a grid cell cluster size consisting of the number of grid cells having the same cluster number as the cluster number that has been assigned to the grid cell. 
     
     
       4. The method of  claim 1 , wherein the grid property data for each grid cell further comprises the number of fractures in the grid cell that belong to the fracture cluster that has the unique fracture cluster number that uniquely maps to the unique grid cell cluster number. 
     
     
       5. The method of  claim 1 , wherein further outputting cluster statistical data including one or more of the group consisting of: number of cell clusters present in the simulation grid and a percentage of grid cells that form part of a cell cluster. 
     
     
       6. The method of  claim 1 , wherein assigning of inter-cluster flow impediment data consists of imposing a lower effective permeability in at least one of the three directions on grid cells that are adjacent to grid cells that do not have the same grid cell cluster number than the computed effective permeability using the fractures that are inside the grid cell and that form part of the fracture cluster that has the same fracture cluster number as the grid cell cluster number of the grid cell whereby ignoring other fractures that are inside the grid cell but that form part of a fracture cluster having a fracture cluster number that is different from the grid cell cluster number of the grid cell. 
     
     
       7. The method of  claim 6 , wherein the lower effective permeability is imposed on the grid cell that separates two neighboring grid cell clusters and that belongs to the grid cell cluster that contains the fewer grid cells of the two neighboring grid cell clusters. 
     
     
       8. The method of  claim 1 , wherein assigning of inter-cluster flow impediment data consists of imposing an inter-cell transmissibility barrier between adjacent grid cells that do not have the same grid cell cluster number. 
     
     
       9. The method of  claim 1 , wherein the effective permeability for each grid cell is computed using an analytical expression of the effective permeability tensor using Oda's method wherein adding weighted fracture permeabilities in each of the three directions. 
     
     
       10. The method of  claim 1 , further comprising creating a field development plan comprising using the table containing the grid property data for all grid cells and the inter-cluster flow impediment data, and subsequently executing the field development plan employing a well that connects into the subsurface fractured hydrocarbon bearing reservoir formation. 
     
     
       11. A computer readable medium having stored on it a set of computer instructions that when loaded in a computer is capable of carrying out steps comprising:
 receiving a discrete fracture network model of a subsurface fractured hydrocarbon bearing reservoir formation, said discrete fracture network model comprising a plurality of fractures as input; 
 grouping all fractures of the plurality of fractures into fracture clusters, whereby all fractures that are physically connected with each other by intersection, either directly or indirectly via a number of other physically connected fractures, exclusively belong to one fracture cluster such that no fracture within one fracture cluster is physically connected with any fracture of any of the other fracture clusters; 
 assigning a unique fracture cluster number to each of the fracture clusters; 
 overlaying a three dimensional simulation grid over the discrete fracture network model, consisting of a three dimensional array of grid cells in three directions; 
 attributing a grid cell cluster number to each grid cell in the simulation grid, which grid cell cluster number uniquely maps to the unique fracture cluster number of exclusively one fracture cluster that intersects with the grid cell; 
 computing an effective permeability for each grid cell and for each of the three directions using the fractures that are inside the grid cell and that form part of the fracture cluster that has the fracture cluster number to which the grid cell cluster number of the grid cell is uniquely mapped, whereby ignoring other fractures that are inside the grid cell but that form part of a fracture cluster having a fracture cluster number that does not map to the grid cell cluster number of the grid cell; and 
 assigning inter-cluster flow impediment data to selected grid cells reflecting flow impediment barriers between neighboring grid cell clusters; 
 outputting a table containing grid property data for all grid cells, wherein the grid property data for each grid cell comprises the cluster number assigned to the grid cell, and one effective permeability number for each of the three directions; and 
 outputting inter-cluster flow impediment data. 
 
     
     
       12. A computer system programmed to carry out steps comprising:
 receiving a discrete fracture network model of a subsurface fractured hydrocarbon bearing reservoir formation, said discrete fracture network model comprising a plurality of fractures as input; 
 grouping all fractures of the plurality of fractures into fracture clusters, whereby all fractures that are physically connected with each other by intersection, either directly or indirectly via a number of other physically connected fractures, exclusively belong to one fracture cluster such that no fracture within one fracture cluster is physically connected with any fracture of any of the other fracture clusters; 
 assigning a unique fracture cluster number to each of the fracture clusters; 
 overlaying a three dimensional simulation grid over the discrete fracture network model, consisting of a three dimensional array of grid cells in three directions; 
 attributing a grid cell cluster number to each grid cell in the simulation grid, which grid cell cluster number uniquely maps to the unique fracture cluster number of exclusively one fracture cluster that intersects with the grid cell; 
 computing an effective permeability for each grid cell and for each of the three directions using the fractures that are inside the grid cell and that form part of the fracture cluster that has the fracture cluster number to which the grid cell cluster number of the grid cell is uniquely mapped, whereby ignoring other fractures that are inside the grid cell but that form part of a fracture cluster having a fracture cluster number that does not map to the grid cell cluster number of the grid cell; and 
 assigning inter-cluster flow impediment data to selected grid cells reflecting flow impediment bathers between neighboring grid cell clusters; 
 outputting a table containing grid property data for all grid cells, wherein the grid property data for each grid cell comprises the cluster number assigned to the grid cell, and one effective permeability number for each of the three directions; and 
 outputting inter-cluster flow impediment data.